1. Field of the Invention
This invention relates to methods of making stents from bioabsorbable polymers.
2. Description of the State of the Art
This invention relates to radially expandable endoprostheses that are adapted to be implanted in a bodily lumen. An “endoprosthesis” corresponds to an artificial device that is placed inside the body. A “lumen” refers to a cavity of a tubular organ such as a blood vessel. A stent is an example of such an endoprosthesis. Stents are generally cylindrically shaped devices that function to hold open and sometimes expand a segment of a blood vessel or other anatomical lumen such as urinary tracts and bile ducts. Stents are often used in the treatment of atherosclerotic stenosis in blood vessels. “Stenosis” refers to a narrowing or constriction of a bodily passage or orifice. In such treatments, stents reinforce body vessels and prevent restenosis following angioplasty in the vascular system. “Restenosis” refers to the reoccurrence of stenosis in a blood vessel or heart valve after it has been treated (as by balloon angioplasty, stenting, or valvuloplasty) with apparent success.
Stents are typically composed of scaffolding that includes a pattern or network of interconnecting structural elements or struts, formed from wires, tubes, or sheets of material rolled into a cylindrical shape. This scaffolding gets its name because it physically holds open and, if desired, expands the wall of the passageway. Typically, stents are capable of being compressed or crimped onto a catheter so that they can be delivered to and deployed at a treatment site. Delivery includes inserting the stent through small lumens using a catheter and transporting it to the treatment site. Deployment includes expanding the stent to a larger diameter once it is at the desired location. Mechanical intervention with stents has reduced the rate of restenosis as compared to balloon angioplasty. Yet, restenosis remains a significant problem. When restenosis does occur in the stented segment, its treatment can be challenging, as clinical options are more limited than for those lesions that were treated solely with a balloon.
Stents are used not only for mechanical intervention but also as vehicles for providing biological therapy. Biological therapy uses medicated stents to locally administer a therapeutic substance. A medicated stent may be fabricated by coating the surface of either a metallic or polymeric scaffolding with a polymeric carrier that includes an active or bioactive agent or drug. Polymeric scaffolding may also serve as a carrier of an active agent or drug.
Furthermore, it may be desirable for a stent to be bioresorbable. In many treatment applications, the presence of a stent in a body may be necessary for a limited period of time until its intended function is accomplished, for example, maintaining vascular patency to allow for remodeling of the vessel walls at an increased diameter. Therefore, stents fabricated from bioresorbable, bioabsorbable, and/or bioerodable materials, such as polymers, should be configured to completely resorb only after the clinical need for them has ended.
However, one of the challenges of making medical devices out of polymers is that properties of a polymer can deteriorate both during processing and after processing. These properties include mechanical properties such as strength and toughness and bioresorption kinetics. The processing steps in a fabrication process of a stent may be designed to maintain or instill in the stent particular ranges of the above properties that are crucial for treatment with the stent. Medical devices such as stents are typically stored for an indefinite period of time after fabrication is completed during which properties of the polymer can change as a function of time away from such desirable or crucial ranges. Therefore, methods are needed that reduce or eliminate undesirable changes in properties.